The use of underground pipes to carry fluids and gasses is prevalent throughout the world. Typically, these pipes are utilized to carry water, natural gas, etc. During installation of the pipes, flaws may occur in the pipe. One common flaw, for example, that would be seen as an installation flaw would be a bad weld between two connected pipes.
Not only do installation errors occur which lead to flaws in the pipe, but the pipe can develop flaws over the period of time it is in the ground. For example, a significant portion of the piping used for underground fluid transfer is comprised of a metal alloy. When exposed to damp conditions, the metal alloy has a tendency to corrode at certain locations throughout the pipe. If the pipe corrodes all the way through the wall of the pipe, the fluid being transferred will eventually escape. In the case of natural gas being transferred by a pipe having an entire wall corroded, a dangerous situation (for example, an explosion) may result.
Not only will the pipe have a tendency to corrode, but the pipe may also experience cracks. These cracks may be due to, inter alia, shifting of the ground where the pipe is located or a significant amount of weight placed on the ground directly above the location of the pipe. While the latter condition is less of a problem when there is high overburden (i.e., the amount of soil between the ground surface and the pipe), significant weights may still create very small cracks throughout the length of the pipe. Even these small cracks may present the dangerous situation described above.
Prior art methods to determine the location and/or magnitude of a point of corrosion or a crack in underground piping exist. For example, in U.S. Pat. No. 5,087,873 to Murphy et al., a sinusoidal current is introduced into the pipe and magnetic sensors placed along the length of the pipe detect the resulting sinusoidal magnetic fields emanating at various locations of the pipe. Based on the detected magnetic fields, corrosion points along the length of the pipe can be determined. This method is impractical, however, since access to the pipe to apply the sinusoidal current is required. Also, a plurality of magnetic sensors must be located throughout the length of the pipe. Consequently, this method is implementation and cost prohibitive.
Other methods to determine the location and magnitude of a corrosion point or a crack in underground piping exist. For example, as described in U.S. Pat. No. 4,613,816 to Zeamer, a magnetic probe capable of detecting the location of a magnetic anomaly is implemented. A superconducting quantum interference device (SQUID) is coupled to gradiometer coils and a magnetometer, and detects the magnetic fields sensed by the magnetometer and the gradiometer coils to determine the intensities, directions and gradients of the article under consideration. A SQUID is a relatively new technology for inspection of underground piping such as gas transmission lines. SQUIDs are described in U.S. Pat. No. 5,293,119 to Podney, which is incorporated herein by reference in its entirety. SQUIDs provide unprecedented sensitivity at ultra low frequencies (.about.10 Hertz), to enable inspection of buried pipelines from the surface. Their magnetic flux resolution of 10.sup.-6 .PHI..sub.o /.sqroot.Hz enables a magnetic telescope to measure leakage of magnetic flux from pipeline flaws through a 2 m overburden, where a quantum of magnetic flux, .PHI..sub.o, is 2.07.times.10.sup.-15 Webers. The detection of small flaws, however, is severely limited by physical properties of the magnetic telescope itself.
For example, the magnetic telescope typically is the source of the magnetic field which is detected by the SQUID. The source, however, significantly contributes to the amount of interference seen by the SQUID. In order for the SQUID to achieve the sensitivities that it is capable of, the SQUID must be in as "quiet" an environment as possible. In other words, noise suppression at the location of the gradiometers and the SQUIDs must be optimized. By optimizing the noise suppression at the location of the gradiometers and the SQUIDs, smaller corrosion patches or cracks may be detected for the same amount of source energy which is output by the magnetic telescope. This in turn has the advantage of locating corrosion early, or cracks while they are still small, thus substantially mitigating the dangerous effects that such corrosion or cracks may eventually produce.
Thus, a need exists for a magnetic telescope which overcomes the interference presented by its own source by enhancing noise suppression so that the sensitivity of a SQUID, as used therein, is increased.
It is therefore an object of the present invention to provide improved noise suppression in a magnetic telescope in accordance with the invention.
It is another object of the present invention to provide improved noise suppression by implementing a geometric and electronic configuration in a magnetic telescope.
A related object of the present invention is to provide a differential source coil and gradiometer geometric configuration to enhance noise suppression in a magnetic telescope.
Still another related object of the present invention is to provide an electronic configuration by adjusting the currents in the source coils and balancing the magnetic flux in the gradiometers.
Another object of the present invention is to provide an improved dewar in which the magnetic telescope resides during use.
A related object of the present invention is to provide an improved dewar which has improved cryogenic efficiency with a compact, small, lightweight design.